[gambit-list] Nearly 20% performance improvement with -fstrict-aliasing

Bradley Lucier lucier at math.purdue.edu
Thu Oct 25 16:26:48 EDT 2012


On Thu, 2012-10-25 at 16:19 -0400, Bradley Lucier wrote:


> This has the following effect on the time to compute 10,000 forward
> and 10,000 backward complex FFTs of 1,024 elements.
> 
> With -fno-strict-aliasing (used now by Gambit for reasons I'll explain
> below):
> 
> (time (do ((i 0 (fx+ i 1))) ((fx= i 10000)) (direct-fft-recursive-4 a
> table) (inverse-fft-recursive-4 a table) (do ((i 0 (fx+ i 1))) ((fx= i
> (fx* two^n 2))) (f64vector-set! a i (fl/ (f64vector-ref a i)
> inexact-two^n)))))
>     705 ms real time
>     704 ms cpu time (704 user, 0 system)
>     6 collections accounting for 2 ms real time (0 user, 0 system)
>     1600000 bytes allocated
>     22 minor faults
>     no major faults
> 
> With -fstrict-aliasing:
> 
> (time (do ((i 0 (fx+ i 1))) ((fx= i 10000)) (direct-fft-recursive-4 a
> table) (inverse-fft-recursive-4 a table) (do ((i 0 (fx+ i 1))) ((fx= i
> (fx* two^n 2))) (f64vector-set! a i (fl/ (f64vector-ref a i)
> inexact-two^n)))))
>     590 ms real time
>     592 ms cpu time (588 user, 4 system)
>     6 collections accounting for 2 ms real time (4 user, 0 system)
>     1600000 bytes allocated
>     22 minor faults
>     no major faults
> 
> That's a 19% performance improvement.  (And the performance isn't bad,
> 1.74 GFLOPS on my 2.33GHz Core 2 quad 6600.  Later I'll try to get gcc
> to vectorize the main loop.)


I should have said which OS, gcc, and configuration I was using:

heine:~/programs/gambit> gsc -v
v4.6.6 20121013190232 x86_64-unknown-linux-gnu "./configure 'CC=gcc
-march=native -fschedule-insns -frename-registers'
'--enable-single-host' '--enable-multiple-versions' '--enable-shared'"


heine:~/programs/gambit> gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.7/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro
4.7.2-2ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-4.7/README.Bugs
--enable-languages=c,c++,go,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.7 --enable-shared --enable-linker-build-id
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.7
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--enable-gnu-unique-object --enable-plugin --enable-objc-gc
--disable-werror --with-arch-32=i686 --with-tune=generic
--enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.7.2 (Ubuntu/Linaro 4.7.2-2ubuntu1) 

model name	: Intel(R) Core(TM)2 Quad  CPU   Q8200  @ 2.33GHz

Brad
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